Fiber optic technology has come into relatively wide usage which continues to expand. Fiber optics are used in such applications as telephone systems, digital data links, local area networks and, to a more limited degree, video transmission. Fiber optic technology also finds use in illumination, medical applications and displays. However, unlike the earlier-mentioned applications, these latter do not usually require switches to link different optical fibers with one another.
Switching of fiber optic signals has proven to be something of a significant technological hurdle. Indeed, the text Understandinq Fiber Optics, .COPYRGT.1987 by author Jeff Hecht and published by Howard W. Sams & Co., states that "optical-switching technology is difficult"and ". . . optical switching is a major problem."
One type of optical switch is shown in U.S. Pat. No. 4,491,384 (Yamashita et al.) and uses a single laser beam coupled into a waveguide. Either or both of a pair of surface acoustic wave (SAW) transducers "operate"on the beam and the interaction of an SAW and the beam serves to direct the beam. Grating lenses further direct a beam and couple it to an optical coupler on the output side of the switch.
By selecting either one or both of two interdigital transducers (IDT) to produce an SAW and by selecting the frequency of the voltage applied to each IDT, the beam can be directed to any one of the couplers in sequence --but not to more than one simultaneously. For example (and as shown in FIG. 1), if only SAW2 is propagating, the beam follows path D to the center coupler. If only SAW1 is propagating, the beam follows path F to the lower coupler. If both SAW1 and SAW2 are propagating, the beam follows path E to the upper coupler. The diffraction angle(s) can be varied by selecting different IDT frequencies, presumably to direct the beam to yet other unillustrated couplers. In the Yamashita et al. embodiment shown in FIGS. 3-5, the input laser beam is split by waveguides to two beams and surface acoustic waves operate singly or in tandem on each beam.
In the second embodiment, two couplers can receive a single beam only by first splitting the beam. It appears that the number of output couplers which can receive a particular beam is equal to the number of "input beams," either one or, if the beam is split, two. To put it another way, it is not possible for more than two or all of the output couplers to simultaneously receive the same beam signal. Nor does the Yamashita et al. patent suggest whether control of the switching function is at the input side of the switch, at the output side or elsewhere. And each coupler is in the path of a beam only when that coupler is actually "receiving"the beam, i.e., in communication with it.
Another type of optical switch is shown in U.S. Pat. No. 3,990,780 (Dakss) and uses plural input and output waveguides and a separate "integrated beam deflection element"adjacent to each such waveguide. The element illustrated is an IDT to generate an SAW. No single input can be simultaneously received by plural outputs and each output is in the path of a light beam only when that output is actually receiving the beam. Two IDT's are required to "route"a single beam to a single output waveguide.
U.S. Pat. No. 4,394,060 (Verber et al.) shows a system for scanning or "sweeping"a light beam, presumably across some area. U.S. Pat. No. 4,013,000 (Kogelnik) shows an optical crossbar switching network in which two layers of parallel strip-like waveguides are arranged in an angular orientation one atop the other to provide a matrix of crossing points. Input light follows the strip along which it enters or, if couplers are actuated, is routed to the adjacent, angularly oriented strip. Switches of this type are known to require complex control systems and tend to be susceptible to "cross-talk."
Other types of optical switch devices are shown in U.S. Pat. Nos. 4,148,556 (Sauter et al.) and 4,425,024 (Keil et al.). The scanner device described in a paper titled "Electrooptic Fresnel Lens-scanner with an Array of Channel Waveguides"by K. Takizawa, published by Applied Optics. Vol 22, #16, Aug. 15, 1983, uses a number of "channel"waveguides having the same length but differing widths with respect to one side of a center line. The waveguides on the other side of the center line are arranged in mirror image. Varying values of DC voltage are applied to the structure to create an electric field used both for focusing and scanning. That is, it exhibits characteristics of a lens and of a scanner. FIG. 6, items (d) and (e) from a paper titled "High Speed Optical Time-Division and Space-Division Switching"by M. Sakaguchi et al. shows types of guided wave, directional coupler switches.